1. Field of the Invention
This invention relates to a mold for continuous casting which has a wear-resistant, flame-sprayed coating film formed on the inner surfaces thereof.
2. Description of the Prior Art
As is known in the art, molds adapted for use in continuous casting and made, for example, of steels include a matrix mold formed of copper or its alloys and a protective layer formed on the inner wall surfaces thereof and made of a wear-resistant material. The protective layers recently proposed, for example, in Japanese laid-open Patent Application Nos. 63-35762 and 1-186245 are ones which are made of a nickel plated layer or a copper plated layer formed on the inner surfaces of the mold and a flame sprayed coating film formed on the plated layer by flame spraying a wear-resistant spraying material such as a Ni-based self-fusing alloy powder or a metal carbide composite material. Another type of protective layer has also been proposed such as in Japanese Laid-open Patent Application No. 62-227554 wherein a nickel simple element which has good miscibility with copper is sprayed against the inner wall surfaces of a mold according to a high-velocity oxygen fuel coating method (hereinafter referred to simply as "HVOF") to form a sprayed coating film, and further forming, on the thus formed sprayed underlying film, several layers of a metal carbide composite material such as cobalt-containing tungsten carbide or its mixture with nickel by flame spraying so that the content of the metal carbide composite material increases from the side of the mold surface toward the side of the protective layer surface.
Where a sprayed coating film is directly formed on the surface of a copper or copper alloy mold according to the flame sprayed coating, it is necessary that the bond strength between the sprayed coating film and the copper or copper alloy be increased. To this end, usual practice is to subject the mold surfaces to blasting prior to the spraying and then to form a film on the blasted surfaces by sprayed coating of a wear-resistant material. The blast treatment is a kind of surface treatment which makes use of steel grits or alumina grits as a blasting material. The grits are collided against the matrix surfaces by entrainment with air under pressure to eliminate contaminants and oxide films such as Cu.sub.2 O from the surfaces and also to make coarse irregularities on the matrix surfaces. When a sprayed coating film is formed after having subjected the mold surfaces to the blast treatment, fused particles are anchored in individual irregularities on the matrix surfaces thereby permitting the film to have greater bond strength.
However, when a copper or copper alloy matrix is subjected to the blast treatment, the steel or alumina grits used as a blasting material are liable to remain on the inner wall surfaces of the blasted mold, coupled with another problem that a residual stress may develop in the inner wall surfaces of the mold and the inner wall surfaces may become weakened. This is a serious problem involved in the blast treatment of copper and copper alloys. It will be noted that although not specifically set out in the afore-indicated Japanese Laid-open Patent Application No. 62-227554, where nickel simple element miscible with copper is spray-coated according to the HVOF method to form an underlying layer in order to enhance the strength of bonding with copper or copper alloys, the blast treatment is necessary for the elimination of oxide films from the mold surfaces and for the roughening of the surfaces.
On the other hand, the sprayed coating film is formed on known continuous casting molds according to a spraying method called HVOF wherein the working pressure of a spray gun is not higher than 0.5 MPa (mega pascal), under which the spraying material is fully fused by means of the heat from a spraying flame, making it difficult to intensely anchor the material in the copper or copper alloy substrate (hereinafter referred to simply as "copper substrate" or "substrate"). Accordingly, the bonding force between the substrate and the sprayed coating film is low, and the substrate in the sprayed condition does not stand use.
To avoid this, the known spraying methods essentially require that after completion of the sprayed coating, a high temperature heating treatment (i.e. fusing treatment) at about 900.degree. to 1000.degree. C. be essentially required in order to form a more dense film and to improve the bond strength owing to the formation of the diffusion layer between the sprayed coating film and the substrate (Cu or (Cu+ metal plating)).
However, where the copper substrate, which is made of a non-precipitation hardening material (e.g. pure copper such as deoxidized copper), is subjected to the fusing treatment at about 900.degree. to 1000.degree. C. after the spraying, there arises the problem that the copper substrate considerably lowers with respect to its inherent strength. More particularly, non-precipitation hardening substrates whose strength has been increased owing to the stress caused by cold working have the vital problem in that when they are inevitably recrystallized, the material strength lowers considerably, making it impossible to use such substrates as a casting material. Under these circumstances, it has been usual to use precipitation hardening substrates which are able to restore the strength thereof for use as a casting material through thermal treatment.
In prior art methods where a precipitation hardening material is used as a copper substrate, it is necessary for the reasons set out hereinbefore that the substrate after spraying be subjected to fusing treatment at about 900.degree. to 1000.degree. C., then quenched and finally thermally treated at about 400.degree. C. for precipitation hardening treatment. However, the quenching (with water) of the copper substrate which is to be effected primarily as a solution treatment for restoring the material strength of the copper substrate has the problem that the difference in the coefficient of thermal expansion between the sprayed coating film and the copper substrate is so great that the sprayed coating film may separate or may suffer cracking therein. In this sense, the quenching is in fact impossible and the subsequent thermal treatment cannot be performed to a satisfactory extent. Eventually, this also leads to the vital problem that the copper substrate is not satisfactory with respect to the material strength on use as a mold.
Thus, the known flame sprayed coating methods are disadvantageous in that an additional number of working steps such as the blast treatment prior to the spray coating, the thermal treatment after the spray coating and the like are required and that the working conditions are complicated, resulting in high manufacturing costs. Moreover, in order to keep the wear resistance, the sprayed coating film should be thicker. According to the known HVOF, the film thickness is usually in the range of approximately 0.5 to 1 mm in maximum. A greater thickness involves the problem that the film suffers cracking.